1. Field of the Invention
An arc welding process for butt welding of parts, especially designed for orbital butt welding of tubes for piping applications.
2. Brief Description of the Prior Art
1. Preliminary
Welding has been used for many years. As it is well know in the art, when butt welding parts, the items to be welded must be held in position relative to one another. In the case of the parts being tubes welded together, a clamp is used for this purpose, generally placed inside the tubes and positioned at the junction of the two tubes. This clamp is equipped with rows of pistons, each row holding a tube.
The ends of the parts to be welded may first be chamfered 50 (depending on the thickness of the parts, the arc welding process used, the procedurexe2x80x94automatic or manualxe2x80x94and the like), to form ring 13. See FIGS. 1 and 2 for illustrations of the prior art components that are welded together.
2. Welding Principle
The process used for butt welding parts of the present invention is the MIG/MAG or GMAW metal arc welding with gas shield process. A metal wire 25 is connected at the weld torch 11 to one of the polarities of a power source and is fed to the welding location. An electric arc is created between this wire and the parts to be assembled, which parts are at the polarity different from that of the wire polarity (See FIG. 5 for an illustration of the prior art equipment and process discussed in more detail below).
The electric arc causes the wire (sometimes referred to as the xe2x80x9cfiller metalxe2x80x9d) to melt, as well as partial melt of the parts to be assembled. The wire feed rate (called Vf) is made equal to its melt rate so as to provide for a stable arc. The first weld bead, which produces a perfect joint at the inner skin (with admissible imperfections according to implemented standard) between two chamfered parts is know as xe2x80x9cthe penetration pass.xe2x80x9d
The current wave delivered by the generator is either continuous or sequential. Arc welding is a periodic phenomenon whose period is a few hundredths of a second. During the welding process, the electrical parameters therefore also vary in a periodic fashion.
The current and voltage waveforms of the welding arc change according to the type of transfer of the weld metal from the filler metal (See FIGS. 3 and 4).
3. Types of Generators Used
MIG/MAG arc welding generators capable of generating sequential type arc modes (including the pulse and/or controlled short-circuit modes) illustrate the ability to have controlled waveforms as shown in FIGS. 3 and 4.
Parameters adjustable by the operator on such generators as shown in FIGS. 3 and 4 are:
Ipeak 1, Ibase 2, Upeak 3, Ubase 4, peak time 5, up ramp 6, down ramp 7 (depending on source manufacturers), Vf, frequency (inverse of cycle 51) (adjustable or self-regulated), cool time 8 (adjustable or self-regulated).
In automatic welding, overall parameters managed by the program logic controller (xe2x80x9cPLCxe2x80x9d) are:
Welding unit speed (equivalent to welding speed), oscillation amplitude and frequency of the welding torch.
For Sequential mode: pure pulse mode (FIG. 3), the pulse phase corresponds to the Ipeak dwell time 5.
For Sequential mode: controlled short-circuit (FIG. 4), Upeak: peak voltage. Voltage value 3 at which the current stops increasing, i.e. voltage at the inception of Ipeak 1. Umax 9: maximum achieved voltage. This is a phenomenon due to system inertia.
The pulse phase starts when the voltage becomes equal to Upeak and ends at the end of Ipeak.
In such generators, an intelligent system (microprocessor+programmable logic device, for example) allows certain parameters (Ipeak, frequency, cool time, and the like) to be adapted in real time according to the weld pool conditions (thermal emissivity of the weld pool, for example).
4. For Tube Welding (Penetration Pass), Several Techniques are Available
None of the techniques presented below makes use of real-time, self-assessment of the penetrating power of the electric welding arc.
a. Welding from the Inside:
An inner welding clamp is used. The clamp is usually equipped with two rows of pistons to hold the tubes in position. Further, mobile welding torches are mounted on the clamp for the purpose of completing the penetration from the inside. Parameters can be changed to fit predefined angular positions. The problem with this are that implementing this device is a complex process (positioning in the mating plane is difficult, centering of the welding torches, no direct check possible during welding, poor cost-effectiveness, complex machine with limited diameter range, and the like).
b. Welding from the Outside:
(1) For high wire feed rates, e.g. in the case of mechanized or automatic welding where the welding head movements are controlled by a carriage-type electromechanical assembly, the weld pool must be maintained using backing strips (copper, ceramics, and the like) to prevent the weld pool from collapsing. The backing strips are slaved to the piston rows and applied flat against the back of the joint to be welded when the pistons extend. During welding, parameters can be changed to fit predefined angular positions. The problem is that regardless of the type of medium, the backing strips gradually deteriorate as welding passes accumulate with current MIG-MAG type arc welding processes.
(2) In manual welding, or in automatic welding at low wire feed rates, the outside can be welded without backing strips. The welders are generally assisted by an operator who is requested to adjust the mean welding current according to the weld pool behavior. The problem is that low output occurs, with a welder-dependent process.
In the prior art, whether welding from the outside or from the inside, the welding parameters can be regulated according to the weld pool conditions (temperature, luminosity . . . etc.) or the bevel shape (width, gap, high-low . . . etc.) to control the penetration. To do so, some sensors (thermographic vision, laser, camera, pictures/vision analysis and the like) are available and can be used to measure the thermal evolution of the weld pool, the bevel shape evolution, and the like in real time while welding.
But these devices which need to be handled by the welding system are more cumbersome, fragile, expensive, and less time responsive to adapt the parameters.
5. Example of Configuration of Automatic Outside Welding
As shown in FIGS. 5 and 13, a motor-driven welding unit 10 is mounted on fixed tube 12 by means well known in the art. It is connected to a welding generator 14 and a wire reel 15. It is equipped with one or several welding torches 11 and moves along a ring 13 integral with the fixed tube 12, the welding unit 10 welds, for example, a half-circumference of ring 13.
6. General Problems Experienced When Completing the Penetration Pass
a. A Requirement Exists to Control Weld Pool Fluidity in all Positions (Orbital Welding).
Fast cooling is controlled by varying the welding arc heat input. With no support, or with a support made of a low thermal conduction material, cooling of the weld pool during the penetration pass is slower than on a metal support (such a copper). The weld pool must therefore otherwise cool off fast enough to limit the effect of gravity on the fluid pool, thus preventing it from collapsing and avoiding gaps, overpenetration and concavity.
b. The Electric Arc Penetrating Power Varies as a Function of Angular Position.
Even under perfect pipe fit-up conditions (no gaps or high-low areas), the penetrating power of an electric arc is not constant. According to the local angular tilt of the interface of the parts to be welded, the position of the weld pool relative to the arc varies. Thus, for example, for tube welding, at the 12 and 6 o""clock positions, the weld pool is located vertically juxtaposed to the arc, as it is pushed away by the arc pressure: in this case, the arc penetrating power into the metal is high.
Conversely, at the 3 and 9 o""clock positions, the weld pool tends to run under the arc, owing to gravity, as the pressure exerted by the arc is not always adequate, according to the pool fluidity, to push the weld pool back. The arc penetrating power is therefore reduced as compared to the 12 and 6 o""clock positions, under comparable fit-up conditions.
c. Penetration is Harder to Achieve on Fit-up Defects (See FIGS. 6. 7. 8 and 9 for Illustrations of Defects).
In the presence of fit-up defects (gap and/or high-low), the welding arc penetrating power naturally increases, and the weld pool cooling rate is slower (as side thermal pumping by the parent metal is less effective), and gaps may be created (owing to the lack of metal or collapse of the weld pool). When using backing strips, the backing strips may be severely damaged as the electric arc comes into contact with them.
d. Penetration is Harder when the Machining Dimensions of the Ends to be Assembled Vary.
The values of the various machined dimensions of the ends to be assembled, essentially those of the part to be fused during the penetration pass (usually referred to as the root face), i.e. the dimensions of the chamfers (FIG. 10) such as root face 21, lip 22, radius 20, must, as far as practicable, need to be accurate so as to ensure the quality in manual or automated welding. In production, however, the machining conditions for chamfers and the critical root face are often less than ideal and defects may occur. Thus, if the root face thickness is smaller and/or if the width of the lip is greater than nominal values, the ability of the parent metal to cool off the weld pool will be lessened and the electric arc penetrating power will subsequently increase, with the additional risk of creating defects such as gaps or collapsing, under substantially constant mean welding arc electrical parameters.
Conversely, if the root face is thicker and/or the lip narrower than the nominal values, the ability of the parent metal to cool off the weld pool will increase and the arc penetrating power decrease, but a risk exists that not all the parent metal will melt (i.e. the total thickness of the root face), thus creating a defect known as lack of penetration.
It is an object of the present invention to control the weld pool by simple sensory readings and to adapt the welding parameters to changes in the weld pool conditions.
It is a further object of the present invention to have such control using reliable sensors to consistently regulate in real time the weld pool conditions through the welding parameters.
A process for real-time assessment of the penetrating power of the electric welding arc during a butt weld penetration pass is disclosed. The penetrating power is defined as the ability of the electric arc to fuse the metal to be melted (usually referred to as the parent metal).
The purpose of the invention, based on real-time analysis of the welding arc waveforms (current, voltage), is to enable real-time assessment and adjustment of the welding arc penetrating power during the butt weld penetration pass.
The penetrating power changes constantly during the welding phase, and until now, welding current sources and waveform programming only took into consideration a mean penetrating power estimated for constant or steady state welding conditions (fit-up, chamfers). However, these conditions may change locally. Thus the piercing or collapse or burning through phenomenon is due to excessive welding arc penetrating power under given fit-up and local weld pool fluidity, and is detectable through real-time analysis of electrical parameters. Likewise, the lack of penetration phenomenon is due to a low welding arc penetrating power under given fit-up and local weld pool fluidity conditions, and is also detectable through real-time analysis of electrical parameters.
Both burning through and lack of penetration are unacceptable defects which require repair. Sequential modes provide for different weld pool fluidity and penetrating powers (high velocity of the metal transferred during the Ipeak current) according to programming of the waveform parameters (current, voltage, frequency, and the like). However, for given programmed parameters, stabilizing penetration is difficult, owing to the critical character of the thermal balance (heat input/removal) obtained under good fit-up and chamfer conditions, and to the differences of power levels necessary to achieve correct penetration under poor fit-up conditions or with poorly machined chamfers.
The invention will enable the arc mode and all the overall parameters of the installation to be slaved in real time so as to regulate the arc penetrating power and achieve constant penetration without backing or with backing (ceramics, copper, and the like) whether joint preparation is perfect or exhibits imperfections through the use of the voltage and the current of the welding machine.